My invention relates to an improved power supply circuit for a microwave oven and, more particularly, to an improved adjustable supply for controlling the average microwave power generated by the oven magnetron.
The microwave oven is a known appliance used to heat or cook food by exposure to microwave energy. The microwave energy is generated by a microwave tube, typically a magnetron, which converts DC power to high frequency microwave energy, usually at 915 or 2,450 megahertz. In turn, the DC energy is supplied through the power supply which converts the line voltages and frequencies, 120-volts 60-hertz AC found in the home, to the higher voltage levels, typically on the order of 3,000 to 4,000 volts required for operation of available magnetrons. In early microwave ovens the power supplies provided certain fixed levels of voltage which, when matched with a particular magnetron, provided fixed average input powers. By way of example, 650 watts has been an average input level to the magnetron set by the designer's choice. More recent microwave ovens contain a control accessible to the user to permit the average input power to the magnetron to be adjusted to any power between "low" power and "high" power, 300 to 650 watts by way of example. That adjustment allows the user to essentially tailor the cooking speed and power level with the type of food to be heated or cooked. The benefits of this control is known and obvious and need not be discussed here.
An existing circuit for the foregoing type of adjustment appears in the Litton microwave oven manufactured by the Litton Industries, Litton Microwave Cooking Division, Minneapolis, Minnesota, and Model No. 416 sold under the tradename "Vari-Cook". That known control circuit employs a Triac-type controller. For low power levels the magnetron is fed by a pulse of current of predetermined width. At larger power levels the width of this pulse is expanded. I further understand that a switch is provided in the circuit so that when the control is advanced to the full power position, the switch operates to close a circuit to keep the Triac in the On condition. A printed electrical schematic circuitry or explanation of such prior circuit might be available from the manufacturer or a schematic diagram may be derived by inspection of the oven components.
Thus, by way of illustration, if the magnetron is pulsed with a certain peak level of current for a certain period of time, say 100 milliseconds, with a period or time base of 1,000 milliseconds, an average power is obtained which is the power provided to the magnetron over the time period. By increasing the time in which the peak current is supplied to the magnetron within that time base period, such as by doubling the duration of the pulse, say to 200 milliseconds, the average power supplied to the magnetron is effectively doubled.
The aforecited Litton oven of course contains a power supply which includes a high voltage transformer for transforming the line voltage up to the voltage level required for operating the magnetron, contains a separate filament transformer for stepping down the line voltage to the level required by the heater of the magnetron, includes a rectifier circuit for rectifying the AC from the high voltage of the output of the transformer to the DC voltage required by the magnetron and contains the aforedescribed controller in the primary circuit of the high voltage transformer, all of which are known.
In the foregoing oven, the magnetron is operated at its proper input current level for a certain duration and then is turned off for another duration to in effect vary the "duty cycle" of magnetron operation so that the average output power from (and input power to) the magnetron varies as a function of the ratio of On time to the total of On time plus Off time. Two additional prior art circuits as have been made known to me pertinent to this mode of power control appear in patents to Kappenhagen, U.S. Pat. No. 3,332,036, and to Noda, U.S. Pat. No. 3,862,390.
Kappenhagen shows a control for varying the On time of an oscillator with inverse variation of the Off time so that the average power to the oscillator is varied and in which the total time base of On and Off time is essentially constant. Noda is especially pertinent by way of background to my invention in that Noda suggests the use of a pulse generator having a first control to adjust the Off time of the magnetron while maintaining the On time constant--in effect to change the average power input by varying the time base or repetition rate of the pulse generator and a second control for varying the On time of the magnetron while maintaining the Off time constant--similarly in effect to change the average power input by varying the time base, which is equal to the sum of the On and Off times, of the multivibrator type oscillator. Thirdly, it appears recognized that by the manipulation of both controls it is possible to vary both the On time and the Off time of the oscillator in Noda so as to increase the On time and decrease the Off time to obtain a specified power input and to keep the oscillator time base constant in the manner of Kappenhagen. This is obviously impractical inasmuch as the ordinary consumer must act as a computer in an attempt to figure out which combination of the two control settings provides the desired average magnetron power to properly cook the food within a period of time set upon the oven's main timer, which must also be selected.
Thus Noda, while showing the possibility of two controls suggests that one be set at a prescribed level. Moreover, while the structure of Noda is cumbersome to operate, it also requires two separate adjustable controls which obviously results in a more expensive structure than otherwise might be available as in my invention and overlooks the combination of elements that I believe promotes the life of the power supply elements such as the power transformer.
While the aforedescribed prior art design of a power supply for permitting variable power levels to the magnetron exists, it is recognized that an improvement to the oven combination is made if the reliability of the combination is enhanced. Moreover, conservation of natural resources is promoted if a design like the present invention is invented which reduces the number of components in the oven controller circuitry while providing the same or improved results.